Granular laundry detergents containing admixed sodium carbonate have been found to exhibit poor solubility under certain conditions. This results in clumps of detergent, which appear as solid white masses ranging from about 5 to 40 millimeters in diameter and about 2 to 10 millimeters in length, remaining in the washing machine and on washed clothes. Such clumps usually occur when the detergent is placed in a pile, particularly during cold water washes and/or when the order of addition to the washing machine is laundry detergent first, clothes second, and water last. It has been found that the primary contributor to this solubility problem is the admixed sodium carbonate in the laundry detergent. While not intending to be limited by theory, it is believed that this solubility problem is caused by hydration of the sodium carbonate, which results in a sticky, poorly soluble mass, before the granular detergent can be dispersed and solubilized in the laundering solution. Surprisingly, granular detergents containing sodium carbonate as part of a spray dried base granule usually do not exhibit this solubility problem, apparently because the carbonate is uniformly dispersed in a matrix of more soluble ingredients. Granular detergents also generally have poorer solubility if they contain sodium pyrophosphate builder.
Sodium carbonate and sodium pyrophosphate have been used in granular detergent compositions (for example, U.S. Pat. No. 4,299,717, Cottrell et al, issued Nov. 10, 1981). Potassium salt has been substituted for sodium salt to eliminate giant micelles of sodium salt of washed fatty acid in the washing liquid (Japanese Patent Application 61164000). The solubility of a solid is lower in a solution of a salt containing a common ion than in pure water (Chemical Principles, 4th ed., Masterton Slowinski, W. B. Saunders Co. 1977, pg. 435). A water-softening composition comprising tetrasodium pyrophosphate and an alkaline material selected from the group consisting of trisodium phosphate, sodium hydroxide, sodium carbonates, potassium hydroxide, potassium carbonates, soap and sodium silicate is disclosed in U.S. Pat. No. 2,381,960, Johnson, issued Aug. 14, 1945.
Citric acid has been used as a detergent builder in liquid laundry detergent compositions (see, for example, U.S. Pat. No. 4,507,219, Hughes, issued Mar. 26, 1985). Citric acid has also been disclosed as a builder for granular detergent compositions (e.g., U.S. Pat. No. 4,169,074, Conrad et al, issued Sep. 25, 1979). However, because granular laundry detergents are typically formulated to provide a wash water pH of about 9.8 to 10.5, salts of citric acid such as sodium citrate are more commonly disclosed in granular detergents (see, for example, U.S. Pat. No. 4,715,979, Moore et al, issued Dec. 29, 1987).
It has now been discovered that the addition of low levels of citric acid to granular laundry detergents containing admixed sodium carbonate improves their solubility in the laundering solution and eliminates or reduces the problem of clumps remaining in the washing machine and on washed clothes. It is believed that the citric acid rapidly reacts with the sodium carbonate in the laundering solution to release carbon dioxide. This helps disperse the detergent and minimize the formation of insoluble clumps.